X-ray has been used to form an image of an object in medical and industrial applications. In one application, an X-ray microscope is an instrument used to produce enlarged images of samples illuminated with X-rays. There are two main types of microscopes: full field microscopes and scanning microscopes. In full field microscopes, the whole field of view is imaged to a detector plane at the same time. In scanning microscopes, the sample is illuminated with a bright well focused spot scanning over the sample.
FIG. 1 schematically shows a conventional X-ray microscope with focusing optical elements. As shown in FIG. 1, the conventional X-ray microscope includes an X-ray point source 101, a focusing optics 104, and a detector for the resulting X-ray image 103 of the sample 102. In practice, the point source is not very small (>5 microns), and thus the resulting image is not very sharp. Therefore, the focusing optics 104 is needed to focus X-ray from the X-ray point source 101 into a tiny virtual source 105. The focusing optics 104 may be a Fresnel zone plate. The magnification of the set-up is the quotient L2/L1 with the distance source-sample L1 and the distance source-image L2.
Fresnel zone plate, like most refractive optics that can be used as the focusing optics 104, has chromatic aberration. Therefore, focal lengths of the Fresnel zone plate are different for X-rays with different wavelengths or frequencies. To avoid chromatic aberration in X-ray imaging, a conventional solution is to use a microfocus X-ray source that is monochromatic. A monochromatic microfocus X-ray source may be produced with microfocus tubes. But this requires a very efficient filter, which is difficult to achieve for X-ray, where useful X-ray photons may be lost.
X-ray computed tomography (CT) is another application of X-ray image, using X-rays to create cross-sections of an object that can be used to recreate a virtual model, e.g. a three-dimension (3D) mode, without destroying the original object. X-ray microtomography or micro-computed tomography (micro-CT) is X-ray CT when the pixel sizes of the cross-sections are usually in the micrometer range.
FIG. 2 schematically shows a conventional X-ray CT system. As shown in FIG. 2, the X-ray CT system in FIG. 2 includes an X-ray source 201, a capillary condenser 202, an objective zone plate 204, a phase plate 205, and a charge-coupled device (CCD) detector 206. The sample 203 is placed at the focal point of the X-ray. This system in FIG. 2 is capable of phase-contrast CT.
Since the capillary condenser 202 can condense X-rays of all wavelengths into a focal point in the nanometer range, there is no need for a filter before the sample. However, an X-ray CT system like this in FIG. 2 is bulky and expensive, and includes a very complicated optical path.